An exhaust gas treatment device of an internal combustion engine, in which an SCR catalytic converter (selective catalytic reduction), which reduces the nitrogen oxides contained in the exhaust gas to nitrogen using the reducing agent ammonia, is used to reduce the NOx emissions, is described in German Patent Application No. DE 101 39 142. The ammonia is obtained from a urea-water solution in a hydrolysis catalytic converter positioned upstream from the SCR catalytic converter. The hydrolysis catalytic converter converts the urea contained in the urea-water solution with water into ammonia and carbon dioxide. To ensure exact dosing, the concentration of the urea-water solution is ascertained. The urea-water solution is brought to a predefined pressure using a pump. A dosing valve-fixes a predefined flow rate. Compressed air is mixed with the urea-water solution in a mixing chamber. The urea-water solution is sprayed together with the added air into the exhaust gas of the internal combustion engine in such a way that a largely uniform flow against the SCR catalytic converter is achieved.
A method and a device, in which a pressurized reagent is also sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter, are described in German Patent Application No. DE 10 2004 018 221 (not previously published). The reagent pressure is fixed at a predefined reagent setpoint pressure as a function of a characteristic quantity. An operating variable of the internal combustion engine and/or a characteristic quantity of the exhaust gas of the internal combustion engine may be used as the characteristic quantity. The predefined reagent pressure setpoint value is regulated during a regulation in which the reagent actual pressure is detected by a reagent pressure sensor. Compressed air may be added to the reagent. The compressed air pressure may also be regulated to a predefined compressed air pressure setpoint value as a function of a characteristic quantity during a regulation, the compressed air actual pressure being detected by a compressed air pressure sensor. A defect of at least one of the pressure sensors may result in reduced performance capability of the SCR catalytic converter, with the consequence that unpurified exhaust gas may reach the environment.
A method and a device, in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter, are described in German Patent Application No. DE 10 2004 044 506 (not previously published). The compressed air is guided via a check valve which has an opening pressure. It provides for diagnostics of the compressed air pressure beginning at a starting instant with the closing of a compressed air regulating valve. A check is performed at at least a second instant to determine whether the compressed air pressure corresponds at least to a lower threshold value, which at least approximately corresponds to the opening pressure of the check valve added to the ambient air pressure. An error signal is provided when the condition is not fulfilled.
A further method and a device, in which a pressurized reagent is sprayed into the exhaust gas of an internal combustion engine upstream from an SCR catalytic converter, are described in German Patent Application No. DE 101 59 849. Fuel which acts as a reducing agent for the NO2 component in the exhaust gas in particular is provided as the reagent.
The present invention is based on the object of providing a method for operating an exhaust gas treatment device of an internal combustion engine, in whose exhaust gas system an exhaust gas treatment device is positioned, and in which a reagent, which is possibly admixed with compressed air, is introduced into the exhaust gas system upstream from the exhaust gas treatment device, and a device for executing the method, which ensure high reliability.